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2026 Edition

ASE L2 Practice Test

Take this free ASE L2 Practice Test to study for your ASE L2 certification exam.

The National Institute for Automotive Service Excellence (ASE) identifies Electronic Diesel Engine Diagnosis Specialist as an individual service area for certification (L2). The following subjects are covered:

• General Inspection 
• Electronic Engine Controls
• Air Induction Systems 
• Fuel Systems
• Emission Systems 

There are 55 questions on the exam, 45 of which are scored, and 2 hours is given to complete it.

To prepare for the ASE L2 exam, use Tests.com's ASE L2 Practice Test. It is written by Dr. John Kershaw, an ASE test question developer and ASE Certified Master Technician. Our interactive test platform allows you to simulate the exam. Take as many practice tests as you need. There's no time limit or recurrent charges. 

To qualifty to take the L2 exam, you must have passed the A9, T2, S2 or H2 exams and passed the A6, T6, S6 or H6 exams. To prepare for the ASE A6 exam, use our ASE A6 Practice Test. To prepare for the A9 exam, use our ASE A9 Practice Test.
 

General Inspection and Diagnosis 

1

A diesel truck equipped with a JWAC (Jacket Water Aftercooler) has low power under load. Which of the following could be the cause?
Incorrect. Please choose another answer.

A leak in the JWAC will raise the intake air temperature, which lowers engine power because the air is less dense and produces less energy.  A is wrong because a leak in the turbo inlet manifold affect oil leakage but not power output.  B is wrong because the off-road Diesel #2 and fuel oil # 2 differ only in the tax applied at the time of sale. Both are dyed red.  D is wrong because EGR has no effect on diesel engine at higher throttle openings, because it is when closed at WOT and open at idle.

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2

A medium duty truck using a HEUI (hydraulically actuated electronically controlled unit injector) fuel system has a misfire code when hot.  Technician A says worn HEUI injectors could be the cause.  Technician B says a shorted glow plug could be the cause.  Who is right?
Incorrect. Please choose another answer.

Tech A is right because when an injector is worn, it will leak fuel internally and reduce power or cause a misfire.  Tech B is wrong because a shorted glow plug will only cause a problem when the engine is cold.

Electronic Engine Controls Diagnosis 

3

The technician gets an infinite ohms reading on the ohmmeter function of the DMM (Digital Multimeter), when measuring resistance in a circuit.  Which of the following could be the cause?
Incorrect. Please choose another answer.

An infinity reading indicates an open circuit because the reading is infinite.  B is wrong because a corroded terminal would cause high resistance not infinity.  C is wrong because with infinite resistance the circuit cannot operate.  D is wrong because infinite resistance indicates an open, so the circuit is incomplete.

4

Technician A says you do not have to solder a repaired wire. Technician B says wire insulation damage is sometimes found where the wire passes through an opening in a metal part.  Who is right?
Incorrect. Please choose another answer.

Tech B is right.  Wire insulation damage can take place when the wire passes through an opening in a metal part.  Tech A is wrong because the standard repair practice is to solder wiring repairs. 

Air Exhaust Systems Diagnosis 

5

Which of the following measuring tools is MOST likely used to measure the temperature of the diesel exhaust gases?
Incorrect. Please choose another answer.
An exhaust pyrometer is the right tool to measure exhaust temperature.  There is no such tool as an exhaust manometer.  Diagnostic Scan tools typically do not measure exhaust temp but may in some applications - it is not the most likely tool to be used.  The MDI (Multiple Diagnostic Interface) is an electronic CAN (controller area network) interface used with a laptop personal computer or diagnostic scan tool for diagnosis and does not measure exhaust temperature.

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6

An active diagnostic trouble code is found on a heavy duty diesel engine with VVT (Variable Valve Timing).  Technician A says the camshaft actuator is opening the exhaust valve too early.  Technician B says a slow response could cause the engine not to meet exhaust emissions.  Who is right?
Incorrect. Please choose another answer.
Tech B is right because the OBD system will detect a malfunction prior to any failure or deterioration in the capability of the VVT system to achieve the commanded valve timing and/or control within a manufacturer-specified time that would cause an engine's NHMC, NOx, or CO emissions to exceed 2.0 times any of the applicable standards or an engine's PM emissions to exceed a threshold of the applicable standard plus 0.02 g/bhp-hr.  Tech A is wrong because the VVT camshaft actuators change intake valve timing not exhaust valve.

Fuel Systems Diagnosis 

7

The engine cranks on a diesel, but will not start. During diagnosis, the ECM (Electronic Control Module) will not communicate with the diagnostic tool. What is the MOST LIKELY cause?
Incorrect. Please choose another answer.
A low supply voltage could result in the ECM shutting down. The ECM could not operate the injectors or communicate with the diagnostic tool.  B is wrong because a failed data link connector could cause the diagnostic tool to be unable to communicate with the ECM, but it would not result in a no-start condition. C is wrong because a failed engine speed sensor or timing sensor would cause the ECM to not operate the injectors, resulting in a no-start problem. Yet, the loss of the speed/timing signal would not cause a diagnostic tool communication problem.  D is wrong because when the diagnostic tool software does not match the ECM being diagnosed, communication will not take place between the ECM and diagnostic electronic service tool.
8

Technician A says the diesel plastic fuel lines are separated by removing a clip and disconnecting (squeezing) the secondary latch on the line fitting.  Technician B says an air leak at the line connecting the tank pick-up to the transfer pump can cause low-power and hard starting.   Who is right?
Incorrect. Please choose another answer.
Both technicians are correct. Tech A is right because plastic lines are typical on late model diesel engines and separated by removing a clip and disconnecting (squeezing) the secondary latch on the line fitting.  Tech B is right because air is compressible and air in fuel will affect fuel flow to the pump which causes hard starting, low-power and unstable engine operating conditions.  Air can lead to damaged injector tips of newer high pressure nozzles. When fuel which would normally cushion the seating of the nozzle valve within the injector is absent, the nozzle valve impacts the nozzle seat with tremendous force damaging the nozzle tip.

Emission Systems Diagnosis 

9 Which of the following tools best measures backpressure restrictions?
Incorrect. Please choose another answer.
A magnahelic gauge or water manometer compares very small amounts of pressure differential. The limits to acceptable backpressure in most diesel engines is 3" of mercury or 14" of water column as measured with a magnahelic gauge or water manometer. Conspicuous performance complaints are noticeable at approximately 30 - 40" of water. For this reason, the exhaust system is designed and must be maintained to minimize exhaust backpressure. Answer A is wrong because the mercury manometer is not sensible enough. Answer C is wrong because the pressure is very small. Answer D is wrong because a vacuum gauge is like mercury manometer and not sensible enough.

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10 The EGR (Exhaust Gas Recirculation) system is disabled under what engine conditions?
Incorrect. Please choose another answer.
During idle, cold weather or engine operation, minimal or no EGR flow usually takes place. During warm-up, condensation of corrosive exhaust can take place in EGR coolers so flow is minimized or disabled during that period. When high coolant temperatures exist, the EGR flow may also be disabled since cooling of exhaust gases in EGR coolers may contribute to overheated engine operation. High altitude or low barometric pressures may also be a condition to disable EGR operation since EGR flow under these conditions could significantly deteriorate combustion quality and engine performance.
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Quality starts with who wrote the material. Our writers:

John Kershaw, ASE Test Prep Expert, is an ASE Certified Master Technician, an auto industry technical instructor and is an ASE Test Question Developer. He is among the first certified by the ASE in 1972.

ASE L2 Topics
The Electronic Diesel Engine Diagnosis Specialist service area involves the following systems diagnosis:

• General Inspection
• Electronic Controls
• Air Exhaust Systems
• Fuel Systems
• Emission Systems
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